1. Field of the Invention
The present invention relates to an electroluminescence device, and more particularly to an electroluminescence device to be used as an emitting material for various display devices.
2. Detailed Description of Relates Arts
An electroluminescence device (hereinafter referred to as an "EL" device) is easily distinguished because it emits light, and durable to impacts because it is a completely solid device. An EL device using ZnS:Mn, which is an inorganic phosphor, has been widely used. Such inorganic EL devices, however, need an applied voltage that is as high as about 200 V to emit light. Therefore, the driving method is complicated. Given these circumstances, organic thin film EL devices using various materials are now under investigation because they permit greatly reduced voltages to be applied. Vincett et al. produced an EL device using a vacuum-deposited film (luminescent film) of anthracene having a film thickness of about 0.6 .mu.m as an emitting material, and obtained a blue visible light which is visible under normal illumination with an applied voltage of 30 V. (Thin Solid Film, 94 (1982) 171). EL brightness of this device, however, is unsatisfactory and needs a high applied voltage. Furthermore, it is not sufficiently efficient for practical use.
An organic EL device displaying high brightness and relatively high emission efficiency upon application of a low voltage of less than 25 V, has also been developed (U.S. Pat. No. 4,539,507). This device is a multi-layer type of electrode/positive hole injection layer/emitting layer/electrode, in which the film thickness between electrodes must be less than 1 .mu.m. Thus, pinholes are readily formed unless the film quality of the emitting layer is satisfactory, and it should be noted that productivity is low. Although blue light is obtained in this EL device when tetraphenylbutadiene is used as the emitting layer, the efficiency of this device in the bright emission range is as low as 0.008 lm/W that it is unsatisfactory. In order to obtain emissions of blue light, it is necessary to use a material having a peak wavelength of 450 to 480 nm. However, it has been difficult to obtain an efficiency higher than 0.05 lm/W., because the sensitivity of the eye to this wavelength range is about two-tenth lower than that to the green range. Nevertheless, the formation of this device of electrode/hole injection layer/emitting layer/electrode was a breakthrough in that it enabled a bright emission to be obtained upon application of a low voltage when the emitting material forming a emitting layer was properly chosen.
Among the multi-layer EL devices mentioned above, a device, using a diamine compound as the hole injection layer and a complex of aluminum with 8-hydroxyquinoline as the emitting layer, has been developed (Appl. Phys. Lett., 51 (1987) 913). This device emits a green light with a brightness of more than 1000 cd/m.sup.2 at a low voltage of about 10 V. It should be noted, however, that also the film thickness between electrodes must be very thin, for example, about 130 nm; and therefore the device has the drawback that pinholes are readily formed unless the quality of the thin film is satisfactory, as in the above-mentioned device.
According to European Patent Application Laid-open No. 0281381 an EL device of a multi-layer type with positive electrode/positive hole injection and transportation zone/emitting zone/negative electrode, is disclosed. The emitting zone of this device is made of a thin film comprising a host material and a very small amount of a fluorescent material doped onto the host material. This EL device emits a strong light in the green-red range when a low voltage is applied. built-in by LB process, one of which is a layer made of a luminescent compound having electron accepting property, and the other, a layer made of a luminescent compound with electron donating property, are disclosed in the specifications of U.S. Pat. Nos. 4672265, 4725531, 4734338, 4741976 and 4775820, etc. The luminescent compounds in these elements have a high luminescence quantum efficiency, and a .pi.-electron system which is sensitive to external perturbations. These compounds permit electrical excitation.
In these EL devices, however, it is necessary that the light-emitting layer comprises two layers, in contrast to a emitting layer made of one layer comprising one specified compound in the previously mentioned formation of hole injection layer/emitting layer.
In these EL devices, the EL is caused by the formation of exciplexes, which takes place between the electron donating and the electron accepting compounds forming the two layers, in the vicinity of the interface between these two layers. Thus, the luminescence performance is strongly dependent on the state of the interface. Therefore, the production conditions are delicate in these EL devices, and the EL is reduced remarkably with the degradation of the interface. Furthermore, the EL of exciplexes has a tendency to shift to a longer wavelength than the photo luminescence of the compound itself, i.e. that of the one-layered emission layer, and thus, the emission of blue light with a short wavelength is so difficult that it cannot be achieved by using the EL of exciplexes.
In the EL devices disclosed in the specifications of U.S. Pat. Nos. 4672265 and 4725513, at least one of the two light emitting layers is a molecular accumulated film prepared by the LB process. The long alkyl chain to be used in the molecular accumulated film has a low heat resistant temperature of about 100.degree. C. and is not stable against heating (Kobunshi Gakkaishi, 36 (1987) 267). Therefore, the abovementioned mono molecular built-up film is damaged during vacuum evaporation of the opposite electrode, resulting in a low production yield of the EL devices. Another serious drawback of this mono molecular built-up film lies in that the long alkyl chains are aligned almost perpendicular to an electrode, forming an insulating layer which greatly hinders the mobility of charges. Thus, the EL performance of this device is not satisfactory, because recombinations of electrons and holes are hindered and it is unsuitable for practical use. The two-layer-structured light emitting layer, to which the insulating layer is added, disclosed in the specifications of U.S. Pat. Nos. 4734338, 4741976 and 4775820, has unsatisfactory EL performance and is unsuitable for practical use because the mobility of charges is hindered. Thus, the formations of the EL devices proposed in the specifications of U.S. Pat. Nos. 4672265 to 4775820 have, at present, unsatisfactory performance and have not made any contribution to technological advancement.
In general, it is believed to be important that the emitting materials forming the emitting layer of an EL device have the property of fluorescence in a solid state. Many conventional fluorescent materials, however, lose their fluorescence efficiency in a solid state while they emit fluorescent light as a solution. Fluorescein, rhodamine-based dyes and cyanine-based dyes, for instance, are non-fluorescent or show only greatly reduced fluorescence in a solid state because of association or some other causes. A material such as perylene usually forms dimers in a well-known crystal form, resulting in a sudden - reduction of fluorescence. Therefore, a material which does not lose fluorescence even in a solid state is desired. As an emitting material satisfying this condition and enabling EL blue light emission, a material having a stilbene-skeleton is disclosed. Using this material, a high brightness of about 80 cd/m.sup.2 was obtained when applying a low voltage of about 20 V (EP 0319881).
As another literature describing an application of a distylylbenzene derivative to an organic this film EL device, there is European Patent Application Laid-Open No. 281381, previously cited. According to this literature, p-bis-(o-methylstyryl)benzene are used as fluorescent material which means fluorescent material in a solution. They are doped in a very small amount into the host material as if they were into a solvent. The thin-layered host material, doped with a very small amount of the fluorescent material, forms the light emitting zone in which the light emits from the fluorescent material. The light emitting zone (light emitting layer) must have injection function (a function which enables the injection of holes from an electrode or the hole injection layer, and the injection of electrons from an electrode or the electron injection layer, upon application of an electric field), transport function (a function to transport holes and electrons upon application of an electric field) and light-emitting function (a function which provides a field in which holes and electrons recombine and relate them to light emission). In the EL device disclosed in this literature, injection function, transportation function and part of light emitting function (recombination) are fulfilled by the host material. A very small amount (about 5 mol %) of fluorescent material is doped onto the host material, because only part of its light-emitting function, namely, to emit light according to recombinations between holes and electrons, is used. This literature discloses nothing about the performance of the emitting layer composed of distyryl benzene derivative which necessitates the above-mentioned three functions and has the function both of the host and the guest as well.
Nothing is disclosed about the performance of the emitting materials forming the emitting layer, either.
Furthermore, 1,4-bis(2-methylstyryl)benzene is described as an example of luminescent compounds in the specifications of U.S. Pat. Nos. 4672265, 4725531, 4734338, 4741976, and 4775320 and Japanese Patent Application Laid-Open No. 37890/1986, etc. Nothing is shown, however, about the EL performance of the devices using this compound. They are inventions, in which the light emitting function is specifically attributed to exciplexes, because the light emission is ascribed to interactions between the two layers, as mentioned previously. No technical disclosure is made about the fact that a thin layer made of a distyrylbenzene derivative may function as the light emitting layer enabling emission of bluish light, not based upon the specified light emitting function, i.e. even when the emitting layer does not have a two-layered structure which enables the EL by the exciple